1. Field of the Invention
The present invention relates to copper alloys and to a method for the manufacture of such alloys. In particular, in a first embodiment, this invention relates to a copper alloy with controlled additions of cobalt, nickel and silicon. In a second embodiment, the invention relates to a copper alloy with controlled additions of cobalt, nickel, silicon and silver. The alloys of the invention are particularly suited to be formed into electrical connectors, lead frames and other electric current carrying components.
A third embodiment of the invention is drawn to a method to process both the alloys of the invention and other copper alloys that contain nickel and silicon. More particularly, this process includes hot working a copper-nickel-silicon alloy followed by multiple annealing steps.
2. Brief Description of Art
There is a need in the marketplace for metal alloys having a combination of (1) good formability; (2) high strength; (3) moderately high electrical conductivity; and (4) good stress relaxation resistance. This combination of properties is particularly important for parts that are formed into various electrical interconnections for use in under-the-hood automotive connectors, multimedia (e.g. computers and consumer electronics) electrical connectors, terminal applications, foils, wire and powders, as well as other products. A number of commercial copper alloys are available for use in these applications, but lack the required combination of properties.
The first recited property, formability, is typically evaluated by a bend test. A strip of the copper alloy at a specified gauge and temper, is bent 90° around a mandrel of known radius. The minimum bend radius (mbr) as a function of strip thickness (t) is then reported as mbr/t. The minimum bend radius is the smallest radius mandrel about which a strip can be bent without cracks visible at a magnification of 15×. Generally mbr/t is reported for both good way bends, defined as the bend axis is normal to the rolling direction, and for bad way bends, defined as the bend axis is parallel to the rolling direction. An mbr/t of up to 4t for both good way bends and bad way bends is deemed to constitute good formability. More preferred is an mbr/t of up to 2.
The second recited property, moderate electrical conductivity, is typically viewed as an electrical conductivity in excess of 40% IACS. More preferably, the electrical conductivity is in excess of 50% IACS. IACS refers to International Annealed Copper Standard that assigns “pure” copper a conductivity value of 100% IACS at 20° C. Throughout this patent application, all electrical and mechanical testing is performed at room temperature, nominally 20° C., unless otherwise specified. The qualifying expression “about” indicates that exactitude is not required and should be interpreted as ±10% of a recited value.
The third recited property, high strength, is viewed as a yield strength in excess of 95 ksi (655.1 MPa) and preferably in excess of 110 ksi (758.5 MPa). As the gauge of copper formed into components decreases and as miniaturization of these components continues, a combination of strength and conductivity for a given temper will be more important than either strength or conductivity viewed alone.
The fourth recited property, good resistance to stress relaxation, is viewed as at least 70% of an imparted stress remaining after a test sample is exposed to a temperature of 150° C. for 3000 hours and at least 90% of an imparted stress remaining after a test sample is exposed to a temperature of 105° C. for 1000 hours.
Stress relaxation may be measured by a lift-off method as described in ASTM (American Society for Testing and Materials) Standard E328-86. This test measures the reduction in stress in a copper alloy sample held at fixed strain for times up to 3000 hours. The technique consists of constraining the free end of a cantilever beam to a fixed deflection and measuring the load exerted by the beam on the constraint as a function of time at temperature. This is accomplished by securing the cantilever beam test sample in a specially designed test rack. The standard test condition is to load the cantilever beam to 80% of the room temperature 0.2% offset yield strength. If the calculated deflection exceeds about 0.2 inch, the initial stress is reduced until the deflection is less than 0.2 inch and the load is recalculated. The test procedure is to load the cantilever beam to the calculated load value, adjust a threaded screw in the test rack to maintain the deflection, and locking the threaded screw in place with a nut. The load required to lift the cantilever beam from the threaded screw is the initial load. The test rack is placed in a furnace set to a desired test temperature. The test rack is periodically removed, allowed to cool to room temperature, and the load required to lift the cantilever beam from the threaded screw is measured. The percent stress remaining at the selected log times is calculated and the data are plotted on semi-log graph paper with stress remaining on the ordinate (vertical) and log time on the abscissa (horizontal). A straight line is fitted through the data using a linear regression technique. Interpolation and extrapolation are used to produce stress remaining values at 1, 1000, 3000, and 100,000 hours.
The resistance to stress relaxation is orientation sensitive and may be reported in the longitudinal (L) direction where 0° testing is conducted with the long dimension of the test sample in the direction of strip rolling and the deflection of the test sample is parallel to the strip rolling direction. The resistance to stress relaxation may be reported in the transverse (T) direction where 90° testing is conducted with the long dimension of the test sample perpendicular to the strip rolling direction and the deflection of the test sample is perpendicular to the strip rolling direction.
One group of commercially available copper alloys commonly used for electrical connectors are copper-nickel-silicon alloys. The alloys are precipitation hardenable and achieve high strength through the presence of nickel silicides as a second phase. One copper-nickel-silicon alloy, designated copper alloy C7025 has a composition of 2.2%-4.2% nickel, 0.25%-1.2% silicon, 0.05%-0.30% magnesium and the balance copper. Alloy designations are in accordance with the Copper Development Association (CDA) or New York, N.Y. Copper alloy C7025 is disclosed in more detail in U.S. Pat. Nos. 4,594,221 and 4,728,372 that are incorporated by reference in their entireties herein.
U.S. Pat. No. 6,506,269 discloses copper alloys having controlled additions of nickel, cobalt, silicon and either magnesium or phosphorous. The patent discloses processing the copper alloy by either a high temperature approach or a low temperature approach. The high temperature approach yields properties that fall short of the target combination of strength and conductivity recited above. When processed by the high temperature approach, Exemplary Alloy 1 is reported to have an electrical conductivity of 51.9% IACS and a tensile strength of 709 MPa (102.9 ksi). When processed by the low temperature approach, Exemplary Alloy 1 is reported to have an electrical conductivity of 51.5% IACS and a tensile strength of 905 MPa (131.3 ksi). However the low temperature approach imparts excessive cold working into the copper alloy which is expected to result in poor formability and poor resistance to stress relaxation. U.S. Pat. No. 6,506,269 is incorporated by reference in its entirety herein.
Copper alloy C7027 has a composition of 0.28%-1.0% iron, 1.0%-3.0% nickel, 0.10%-1.0% tin, 0.20%-1.0% silicon and the balance copper. Copper alloy C7027 is disclosed in more detail in U.S. Pat. No. 6,251,199 that is incorporated by reference in its entirety herein.
Japanese Kokai Hei 11(1999)-222,641 discloses copper alloys having controlled additions of nickel, silicon, magnesium and tin. Optional additions include cobalt and silver.
The electrical and mechanical properties of precipitation hardenable copper alloys are strongly influenced by the method of manufacture of the copper alloy. One process for a copper-nickel-silicon-indium-tin alloy is disclosed in U.S. Pat. No. 5,124,124 and includes the processing sequence of continuous cast, solutionize, quench, cold roll, precipitation anneal.
A different process for a copper-cobalt-phosphorous alloy that may optionally contain up to 0.5% in combination of nickel and silicon is disclosed in U.S. Pat. No. 5,147,469. This process includes the process steps of cast, hot roll, quench, cold roll, solutionize, quench, precipitation anneal, quench, cold roll, anneal and quench.
The U.S. Pat. Nos. 5,124,124 and 5,147,469 are both incorporated by reference in their entireties herein.
There remains a need for copper alloys and processes to manufacture those copper alloys to have an improved combination of properties for meeting the needs the automotive and multimedia industries, as well as others where miniaturization is causing more stringent strength and conductivity requirements to be imposed.